The present invention relates to a vehicle control system, and more particularly to a system for operating a pair of transmission assemblies at substantially the same speed and direction, or at different speeds and directions for steering.
Many systems have been employed to transfer the power of an engine to the track units of a track-type vehicle. These fall mainly into two broad classifications--namely, mechanical systems and hydraulic or hydrostatic systems. The mechanical systems include a plurality of gears, shafts and clutches in order to transfer the power of the engine to the track units. Most of the mechanical systems use a brake, or brake and clutch combination, to lock one of the track units while the other track unit turns to provide a steering function for the vehicle. These mechanical systems necessarily use a large number of parts and an undesirable amount of energy is wasted during braking of one track member during a steering operation.
On the other hand, the hydraulic or hydrostatic systems employ variable displacement fluid pumps and motors for converting the mechanical energy of the engine into fluid energy to drive the track units. These hydraulic units are generally less complicated and bulky than mechanical drive units since many of the gears and shafts of the mechanical units are eliminated. An individual hydraulic motor is often provided for each track unit and steering is accomplished by merely changing the speed of one hydraulic motor relative to the speed of the other one. However, often times the control system thereof is complicated and requires much of the operator's attention and effort to drive and steer the vehicle which detracts from the important control of the implements mounted on the vehicle.
Furthermore, the prior control systems which have provided continuously powered controlled radius pivot steering and spot turning have heretofor been excessively complicated and expensive by virtue of the multiplicity of special valves and related components required. Despite the additional hardware, the results have often been less than optimum owing to the insensitivity or lack of responsiveness of the controls, and the difficulty and effort required to control the vehicle during steering and braking.